The present invention relates in general to a new and useful method and apparatus for protecting inverter semiconductor switches from overcurrent faults, and more specifically to diverting fault current from the inverter switches to a crowbar circuit which is sized to handle the overload.
Certain well-known types of power converters are comprised of one or more converter phase legs, each leg including an even number of semiconductor switching devices connected in series. The central junction of each leg comprises the output of that leg. The phase leg semiconductor switches are turned on and off in a manner which provides an AC output voltage at the converter outputs.
Controlled turn-off devices, such as gate turn-off thyristors (GTOs), bipolar power transistors and power field-effect transisitors (FETs), are widely used for the inverter switches. To reduce losses in the converter and to protect the inverter switches, each switch is associated with a series inductive snubber to limit the rate of change of current with respect to time (di/dt) when turning on and a shunt capacitive snubber to limit the rate of change of voltage with respect to time (dv/dt) when turning off.
Fault conditions in a power converter which result in a large overcurrent flowing in the inverter phase leg switches can be caused, for example, by a short-circuit in the ac load or by a malfunction which turns on simultaneously all of the switches in a phase leg. This overcurrent is detected and, in response, the inverter switches must be turned off as quickly and as soon as possible. However, the current that can be turned off by gate turn-off tyristors or other power semiconductor switching devices is limited, and any attempt to turn off a higher current will destroy the device. If no attempt to turn off is made, the surge current discharging the input filter capacitor of the inverter may exceed the capability of the switching device.
Accordingly, it is a principal object of the present invention to improve the operation of voltage-source inverters.
It is another object of the invention to provide overcurrent fault protection in a manner that does not interfere with normal inverter operation.
It is a further object of the invention to provide a method and apparatus for quickly diverting fault current from an inverter phase leg to a crowbar circuit, thus protecting the phase leg switches from damage due to overcurrents.
It is yet another object to provide inverter overcurrent fault protection with few extra components and with low cost.